1. Field of the Invention
The present invention generally relates to brake fluid pressure control devices, and in particular, to a brake fluid pressure control device which functions to control brake fluid pressure in a hydraulic circuit connected to wheels of an automotive vehicle.
2. Description of the Related Art
As disclosed in Japanese Laid-Open Patent Application No. 4-218458, there is known a brake fluid pressure control device that electrically controls a brake fluid pressure supplied to a wheel cylinder. Such a brake fluid pressure control device is provided with a master cylinder which generates a brake fluid pressure (hereinafter known as the master cylinder pressure P.sub.M/C) in accordance with a brake pedal pressure, a brake pedal sensor which detects the brake pedal pressure, a P.sub.W/C sensor which detects the brake fluid pressure (hereinafter called the wheel cylinder pressure P.sub.W/C) supplied to the wheel cylinder, and a high pressure source which is controlled on the basis of the value detected by the brake pedal sensor and the detection value of the P.sub.W/C sensor.
When the system is operating normally, the wheel cylinder is in line with the high pressure source. The high pressure source is controlled so that the wheel cylinder pressure P.sub.W/C is a brake fluid pressure equal to the brake pedal pressure multiplied by a required magnification factor. In such a configuration, it is possible for the wheel cylinder to generate a sufficiently large braking force on the vehicle wheel in response to the brake pedal pressure.
When the detection value for the brake pedal sensor and the detection value for the P.sub.W/C sensor do not satisfy the normal relationship, it can be determined that a malfunction has occurred in the system. When a malfunction has occurred in the system, the high pressure source is separated from the wheel cylinder and the master cylinder is brought in line with the wheel cylinder. According to this configuration, when there is a malfunction, the master cylinder pressure P.sub.M/C is supplied to the wheel cylinder so that the wheel cylinder produces a sufficiently large braking force on the wheel.
In a conventional brake fluid pressure control device as disclosed in the above-mentioned publication, when it is determined that there is a malfunction in the system, the electrical control of the brake fluid pressure using the high pressure source is always stopped and manual control is commenced using the master cylinder. However, under conditions when a defect occurs in only one of a plurality of sensors provided in the system, there are many cases in which the normally functioning sensors can be used to perform the electrical control of the brake fluid pressure using the high pressure source. Accordingly, when a malfunction is detected in the system, it is desired to accurately detect the status of operation of the plurality of sensors provided in the system. If the detection of the sensor operation status is provided, it becomes possible to provide a reliable electrical control for the system under a wide range of conditions.